Thin, porous storage phosphor layer



Dec. 20, 1966 R. H. ANDERSON THIN, POROUS STORAGE PHOSPHOR LAYER 2 Sheets-Sheet 1 Filed March 19, 1962 Fig. no

M/VE/VTOR ROBERT H4 ANDERSON s BUCKHORN, CHEATHAM a BLORE ATTORNEYS O CATHODE TO TARGET VOLTAGE (V a: 075m z wm m m azoomw Dec. 1966 R. H. ANDERSON POROUS STORAGE PHOSPHOR LAYER THIN,

2 Sheets-Sheet 2 Filed March 19. 62

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I/VVE/VTOI? ROBERT H. ANDERSON E M T BN N .AIHA m SR PHOSPHOR THICKNESS BUCKHORN, CHEATHAM 8 BLORE ATTORNEYS United States Patent 3,293,473 THIN, PQROUS STORAGE PHOPHOR LAYER Robert H. Anderson, Portland, 0reg., assignor to Tel:- tronix, Inc, Beavertora, Oreg., a corporation of ()regon Filed Mar. 19, 1962, Ser. No. 180,457 11 Claims. (Cl. 31368) The subject matter of the present invention relates generally to electron discharge display devices and in particular to cathode ray storage tubes which receive electrical input signals, store such signals for an indefinite controllable time and reproduce such signals as visual images for direct viewing or as electrical output signals.

The storage tubes of the present invention may be employed in a cathode ray oscilloscope for recording transient signals, in a radar or sonar display device, as a character-writing tube and as a signal delay device to store electrical signals for a controllable time before producing an electrical output signal which is delayed with respect to such input signal.

Conventional storage tubes employ complex targets including dielectric material deposited on a conducting wire mesh such as that shown by A. V. Haeff in US. Patent No. 2,761,089, issued August 28, 1956, and F. H. Harris, in US. Patent No. 2,839,679, issued June 17, 1958. The storage tube of the present invention is less complicated and less expensive to manufacture than conventional storage tubes because it employs a storage target of much simpler construction. The conducting wire mesh of conventional storage tubes usually has the storage dielectric deposited on one side of the wires of such mesh so that such target exerts grid control on flood electrons which pass through apertures in such dielectric coated mesh to a separate phosphor viewing screen. Such tubes are often referred to in the art as the transmission type of charge storage tubes. They are characterized by the storage of a charge-image on the dielectric layer which coats the mesh, and by the use of a second layer, the phosphor viewing screen, to produce a visible image. In contrast, the storage target of the present invention may be in the form of simply a thin layer of phosphor material supported on the face plate portion of the tube envelope over a transparent conductive coating. This phosphor layer serves both of the two purposes of providing storage of the charge-image and production of the visible light image, which formerly required two separate structures in the commercially manufactured transmission types of charge storage tubes.

The layer of phosphor forming the target of the present invention is a continuous uniform layer in its gross macroscopic structure, but the individual phosphor particles which make up the layer are deposited so that they are not in good electrical contact with each other, especially at the vacuum side of the target surface. This layer of phosphor has a somewhat porous or permeable structure which results in desirable storage characteristics. The target layer of the present invention is not continuous in its electrical conductivity in a direction transverse to the tube axis. The term semi-continuous layer will be used throughout this description to refer to the combined viewing screen and storage target layer, and its structure and and properties are described herein. Such semi continuous phosphor layer also has a thickness within a critical range of thicknesses so that it may perform the functions of both charge image storage and light image emission.

Conventional complex transmission types of storage tubes are subject to spreading or shrinking of the bright lines and area elements which make up the stored image. In such tubes the surface of the storage dielectric is charged to one potential in areas where there is a stored image, but is at a different potential at immediately adjacent areas where no part of the image is being stored.

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This results in a potential difference on the storage dielectric surface across the boundaries between the written, stored, portions of the image, and the unwritten, unstored background portions of the storage surface. The potential difference across these boundaries causes currents to flow, which tend to change the charge condition on both sides of the boundaries, and cause the boundaries to move. The edges of a stored line may move into the unwritten region and cause the stored line to broaden until the stored image is destroyed. A change in adjustment of the voltages on certain tube electrodes may cause the opposite effect, in which the edges of the stored image move into the stored area, until the entire image area decreases until the image is erased.

Motion of the stored image edges is especially noticeable where other effects are present in addition to surface conductivity. For example, in the bistable type of charge storage tube, the storage surface may be bombarded with flood gun electrons and may also be subject to some conductivity from the storage surface through the storage layer to the backing plate or mesh. Ion bombardment of the storage surface is also generally present to some degree. These effects aggravate the tendency for the image boundaries to migrate across the storage surface. It has been found in the conventional complex transmission type of bistable storage tube, that careful adjustment of the collector mesh voltage and other critical voltages may maintain the stored image without substantial spreading or shrinking of the stored area. However, this collector mesh voltage must be held below a certain critical value called the fade positive voltage, above which the stored image will enlarge and spread until it covers the storage surface. The collector mesh voltage must also be held above a second lower critical voltage called the retention threshold voltage, below which the stored image will shrink until it is wholly erased. Accordingly, it will be seen that there is a more or less narrow range of collector voltages within which stable image storage can be obtained, without substantial image boundary migration, in the conventional transmission type of bistable storage tube. This voltage range is often called simply the stable range in the storage tube art, and this term will be used hereafter.

In the storage tube of the present invention, where no collector mesh electrode is employed, it has been found that the characteristic stable range of voltage is now a property of the transparent conductive backing plate which lies between the semicontinuous phosphor storage layer and the glass face plate. The stable range of transparent backing plate voltage has the same property of being bounded by an upper voltage and a lower voltage, between which stable storage occurs, without spreading or erasure of the stored image. In the simplified storage tube of the present invention, a possible explanation of the principle of operation is that the backing plate infiuences or participates in the collection of secondary electrons emitted from the storage surface due to the porosity of the semicontinuous target layer. In any event the stable range property is associated with the potential of the blacking plate in the absence of a collector mesh. The term stable range will be used to refer b oth to the range of collector voltage for stable storage in the conventional transmission type of storage tube, and to the stable voltage range of transparent backing plate voltage in the simplified tube of this invention.

In the complex conventional transmission type of bi stable storage tube as well as in the new type of the present invention, the existence of an adequate stable range is of primary importance. A conventional bistable storage tube is usually considered unsaleable if it has a stable range of less than 30 volts. This is necessary primarily because some allowance must be made for a known decrease of stable range during the useful life of the tube. In addition, some range of tolerance must be allowed for inaccuracy in adjusting the voltages applied to the tube. It is also known that the voltages required by the tube change during tube life, especially when the tube is new. In addition, stable range is not easy to measure accurately and some error must be allowed for in testing newly manufactured tubes. In the manufacture of conventional bistable storage tubes, it has been found that the stable range of a large fraction of newly manufactured tubes is less than 30 volts and that these tubes must be discarded, which greatly adds to the cost and price of the remaining saleable tubes. With conventional bistable storage tubes newly manufactured tubes having a stable range in excess of 60 volts are very uncommon.

The storage tube of the present invention has an inherently much higher stable range. Tubes of this type commonly exhibit stable ranges from 60 to 100 volts with some tubes having more than 100 volts of stable range.

During the development of the conventional transmission type of storage tube, it has also been found that while a degree of storage can sometimes be obtained with the collector mesh spaced closely above the target dielectric, an adequate stable range for a commercially successful tube requires that the collector mesh be placed in contact with the surface of the target dielectric. The contacting collector mesh tends to stop the spreading of the stored image boundaries across the target dielectric surface, which results in a greater stable range in the transmission type of bistable tube.

During the development of the tube of the present invention, an alternate experimental structure was tested in which a phosphor storage target was deposited in the form of separate small islands or dots of phosphor on a transparent conductive substrate. This structure exhibited excellent storage stability. Motion of the stored image boundary was stopped by the semicontinuous dot pattern structure of the dielectric layer and a wide stable range resulted.

Still another target structure was tested during the development of the tube and target of the present invention. This target consisted of individual scattered phosphor particles which were distributed over a conductive coating on a glass face plate, with space between the particles so they were not touching. These particles also showed good storage stability, because spreading of the stored image was stopped by the semicontinuous structure of the scattered phosphor layer, similar to the behavior of the dot pattern target.

It is thus apparent that an adequate stable range of storage depends on preventing the stored image from spreading or shrinking due to image boundary migration. All known commercially successful bistable tubes incorporate some specific physical structure which appears to prevent image boundary migration by breaking up the surface of the target layer. Similarly, experiments which have attempted to use a conventional cathode ray tube viewing screen phosphor as a storage target have failed to produce a commercially successful storage tube. Such tubes have not had an adequate stable range of storage, since there was no structure for breaking up the target surface to prevent image spreading. That is to say the smooth, thick, densely packed conventional phosphor layer will not store. The separated square areas formed by the contacting collector mesh in the previously known transmission type of bistable storage tube provide this required surface semicontinuity as does the dot pattern or the scattered phosphor particles in the two experimental tube types tested during the development of the tube of this invention.

It was found that the storage layer described above, consisting of individual scattered phosphor particles, phosphor particles, would produce a brighter image by using more phosphor particles to form the layer so that the wasted space between individual particles was reduced. In a sequence of experimental tubes, increasing amounts of phosphor were used, until it was found that there is an upper limit to the phosphor thickness which can be used and still have an adequate stable range of storage. For P-l phosphor, it was found that inadequate stable range or complete loss of storage generally occurs when more than about 3 /2 to 4 /2 mg. per cm. is used. This upper limit of thickness is roughly /3 to /2 of the thickness used in conventional cathode ray tube P-l phosphor viewing screen. Layers of this thickness appear to the eye to be fairly dense, conventional continuous layers. However, displacement tests of the packing density of P-l phosphor layers show that about half of the thickness of such layers is empty space, and only about half of the volume of such layers is actually occupied by P-l phosphor particles. The porosity of the resulting layers, and the use of thin layers results in the semicontinuous target surface of this invention. This layer is sufliciently discontinuous to prevent spreading of stored images, which appears to be a necessary feature for adequate bistable storage stability.

The meshes in the transmission type of storage tube are usually made by etching perforations in a thin metal sheet by the well known photoetching process. These meshe are stretched tightly on frames in the tubes so that their mechanical resonance frequencies are made higher than some of the low frequency vibrations of the environment which are common when tubes are subject to shock and vibration. When tubes and targets of larger diameter are used, the meshes must be tensioned tighter to establish the same resonant frequency. Thicker meshes must be used to withstand this greater tension without tearing. Such thicker meshes cannot be perforated with the same number of holes per inch as thinner meshes, because of limitations of the photoetching process. As a result, half-tone and bistable transmission types of storage tubes suffer from a decrease in the maximum possible number of lines per inch of resolution as tube diameter increases. In addition, when such target diameters exceed about 10 to 12 inches, flat meshes are no longer practical because of the excessive tension required to maintain flatness and vibration resistance. In larger diameter half-tone transmission type storage tubes, the metal meshe are curved to gain strength from the structural stiffness of their crowned shape rather than from the restoring forces which arise from the displacement of tensioned membranes. Such meshes are made even thicker than flat meshes, both to withstand the metal forming process of stretching the mesh into a curved shape and to provide a thick curved mesh having greater mechanical strength for environmental shock and vibration resistance. The increased thickness of these meshes results in still fewer holes per inch due to limitation of the photoetching process. Accordingly, the resolution of transmission type storage tubes having diameters over about 10' to 12 inches is less per inch than in smaller tubes of this type. In bistable storage tubes of the conventional transmission type, a complex composite target is used in which two metal meshes and a dielectric coating are sandwiched together in contact, forming a layered structure with the dielectric in the middle. Such bistable targets have not been extended to large diameters and have not been found practical for curved targets.

In the simplified target of the present invention, where the storage target employs no mesh, no decrease in lines per inch of resolution occurs with increasing diameter. Targets above about 10 to 12 inches in diameter do not suffer a further loss in resolution since no thick curved mesh is used. Accordingly, the bistable targets of the present invention may be made in large diameters.

The storage tube of the present invention is also structurally stronger than previous tubes so that it is more resistant to mechanical shock and vibration, since it lacks the fragile dielectric-coated mesh of the transmission type of charge storage tube. Also the present storage tube has a better image resolution than conventional tubes when using a small diameter storage target because the pitch of the collector mesh used in conventional targets limits such resolution. Another advantage is that the storage target of the present invention can be made in rectangular shape more easily than conventional mesh type storage targets because it is extremely diflicult to tension a wire mesh uniformly When it is of a rectangular shape and mounted on a rectangular frame.

In addition, the storage tube of the present invention may be made more compact than conventional storage tubes because it can employ an envelope which includes a funnel portion of ceramic material sealed to a face plate portion of transparent glass so that the end of such funnel portion adjacent such face plate may be of a rectangular hape due to the increased strength of the ceramic material which allows sharp corners in the envelope without increased wall thickness and the corresponding wasted face plate area. Another important advantage of this envelope construction is that the graticule scale may be provided on the interior of the face plate and illuminated through the edge of such face plate so that the light-image producing phosphor layer is closer to such graticule to eliminate parallax. A further important advantage of this envelope construction is that it is readily adaptable for use of a conductive wall coating and a conductive transparent face plate coating which may be operated at different voltages, but which approach each other closely at an approximately right-angled internal angle formed where the flat face plate is sealed to the envelope. This provides for a desirable electron lens collimation structure for control of uniform flood gun electron coverage of the storage target. The transparent conductive coating may also pass through the face plate seal to provide convenient connection on the outside of the envelope. Still other important advantages of the present invention include an increase in brightness of the visible image formed on the phosphor layer of the storage target due to the use of a minimum amount of binder in such phosphor layer. Increased image contrast, writing speed and erase speed have also been obtained by improved methods of operation of such storage tube.

Briefly, one embodiment of the storage tube of the present invention may include an evacuated envelope, a Writing beam electron gun structure having horizontal and vertical deflection plates, a flood beam electron gun structure, and a semicontinuous storage target supported on the interior of the face plate portion of the envelope. The storage target may be in the form of a light transparent substrate body of electrical insulative material, such as a flat face plate portion of the envelope, with a light transparent film of electrically conductive material supported on one ide of such substrate body, and a charge storage dielectric layer of light emissive phosphor material supported on such substrate body over such conductive film. The storage layer may be a semicontinuous layer of phosphor having a substantially uniform thickness that is within a critical range of thicknesses over which such phosphor material will store an electrical charge image for an indefinite controllable time under the conditions present in the tube and will emit a visible light image corresponding to such charge image when bombarded by electrons. The envelope may have a rectangular funnel portion of ceramic material with a flat rectangular face plate of glass sealed thereto. A plurality of axially spaced conducting wall coatings may be applied to the interior of the funnel portion of the envelope and provided with electrical connections to the exterior of such envelope so that such coatings may function a electrodes to focus, collimate and collect the primary electrons generated by the flood gun structure and, in some cases, to collect the secondary electrons emitted by the storage target. These wall coating electrodes along With the writing gun, the flood guns and the conductive film of the storage target are all connected to suitable sources of electrical potential so that the storage tube of the present invention functions as a bistable storage tube in that the target voltage on every area element of the surface of the storage layer tends to be held at one of tWo stable states of electrical potential.

It is therefore one object of the present invention to provide an improved storage tube which is simple and inexpensive to manufacture and whose structure is resistant to mechanical shock and vibration.

Another object of the invention is to provide an improved direct-viewing type storage tube having a storage target of simplified construction in which a semicontinuous layer of phosphor material having a substantially uniform thickness within a critical range of thickness is employed both as a storage dielectric for storing the elec trical charge image without image spreading and as a fluorescent material for emitting the visible light image of such direct-viewing storage tube.

Still another object of the present invention is to pro vide a direct-viewing bistable storage tube having a Wide stable range of operating voltages over which the storage target in such tube will function as a bistable storage target.

A further object of the present invention is to provide an improved storage tube having an envelope with a funnel portion made of ceramic material which is sealed at one end to a light transparent face plate portion of glass material so that the funnel end and the face plate may be of rectangular shape along with the storage target supported on such face plate in order to provide a more compact storage tube.

A still further object of the invention is to provide an improved storage target for use in an electronic storage tube, which includes a layer of phosphor material having a substantially uniform thickness that is within the critical range of thicknesses over which such phosphor will perform the two functions of storing an electrical charge image for an indefinite controllable time and of emitting a visible light image corresponding to such charge image, when bombarded by electrons in such storage tube.

An additional object of the present invention is to provide an improved direct-viewing storage tube having a storage target which emits a light image that has good resolution, brightness and contrast over a wide range of different sized tubes having large and small target areas.

A further object of this invention is to provide an improved method of operation of a direct-viewing bistable storage tube in which the contrast of the visible light image emitted therefrom is substantially increased.

Another object of this invention is to provide an improved method of operation of a bistable storage tube in which the maximum writing speed of the electron beam emitted by the Writing electron gun which will result in storage in such storage tube, is increased.

A still further object of the present invention is to provide an improved method of operation of a bistable storage tube by which the erase speed of such tube is increased so that the charge image on the storage target employed in such tube may be removed and such target may be reconditioned for storing another such image in a shorter time.

Additional objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments of the present invention shown in the attached drawings of which:

FIG. 1 is a side view of one embodiment of the storage tube of the present invention;

FIG. 2 is a top view of the storage tube of FIG. 1 with portions broken away to show internal structure;

FIG. 3 is a front view of the storage tube shown in FIG. 1;

FIG. 4 is a diagrammatic view partly in vertical section and showing one embodiment of an electron gun 7 structure which may be employed in the storage tube of FIGS. 1 to 3;

FIG. 5 is a vertical section view taken along the line 5--5 of FIG. 4;

FIG. 6 is a diagrammatic view partly in horizontal section and shows another embodiment of an electron gun structure which may be employed in the storage tube of FIGS. 1 to 3;

FIG. 7 is a vertical section view taken along the line 77 of FIG. 6;

FIG. 8 is a fragmentary horizontal section view taken along line 8--8 of FIG. 2, showing one embodiment of the storage target which may be employed in the tube of FIGS. 1 to 3;

FIG. 9 is a view similar to FIG. 8 of another embodiment of the storage target which may be employed in the tube of FIGS. 1 to 3;

FIG. 10 shows the secondary emission characteristic of a conventional bistable storage target; and

FIG. 11 shows the storage and display characteristics of the phosphor storage target of the present invention.

A preferred embodiment of the storage tube of the present invention is shown in FIGS. 1 to 3 to include an evacuated envelope having a tubular neck portion 16 of glass, a funnel-shaped body portion 12 of ceramic material and a flat rectangular face plate portion 14 of glass. This envelope structure may be similar to that shown in the copending US. application, Serial No. 132,915, now Patent No. 3,207,936, entitled Electron Beam Display Device, filed by W. H. Wilbanks et al., on August 21, 1961. As stated in such copending application, the glass neck portion 10 may be sealed to the ceramic funnel portion 12 by a glass frit seal 16 at the small end of such funnel portion while the glass face plate portion 14 may be attached to the larger end of such funnel portion by a similar glass frit seal 18. The neck portion of the envelope may contain an electron gun structure including a writing gun and a plurality of flood guns which are shown in greater detail in FIGS. 4 to 7. Electrical lead pins 20 may extend through the side of the neck portion 10 to the horizontal deflection plates, the vertical deflection plates and the isolation shield of such writing gun and to the focusing electrode and isolation shield of such flood guns through a flame seal which joins two tubular glass members forming the neck portion of the envelope. Other electrical leads to electrodes in the writing gun and the flood guns may extend through a seal (not shown) at the base end of the neck portion 10 of the envelope and connected to pins 22 in a plastic base 24 which is suitably secured to such end of envelope neck portion.

The direct viewing storage target 26 of the present invention may be positioned and supported on the interior surface of face plate 14 and will be described in greater detail with reference to FIGS. 8 to 11. A plurality of separate electrodes may be provided as spaced wall coatings of a conductive material, such as silver, tin oxide, aluminum orgraphite, on the interior surface of the funnel portion 12 of the envelope. The first electrode wall coating 28 functions primarily as a focusing electrode for the flood electrons emitted from flood guns. It is connected to a suitable source of electrical poten tial through a first connector plug 30 which extends through a hole in funnel portion 12 of the envelope. A second electrode wall coating 32 having a greater length than the first electrode 28 is spaced fro-m such first electrode and electrically connected to the exterior of such envelope by a second connector plug 34 so that it can also function as a focusing electrode. A third electrode wall coating 36 is provided on the interior surface of funnel portion 12 and spaced from the second electrode 32. It is positioned near the storage target 26 and functions primarily as a focusing and collimating electrode for the flood electrons so that such electrons are substantially uniformly distributed over the surface of the storage target 26 and approach such target at approximately right angles thereto. The third electrode wall coating 36 is also connected to a source of electrical potential by -a third plug connector 38. A fourth electrode wall coating 40 is positioned between and spaced from the third electrode 36 and the storage target 26, and is electrically connected to the exterior of the envelope by a fourth plug connector 42 in a similar manner to the other electrode wall coatings 28, 32 and 36. In addition to focusing and collimating the flood electrons this fourth electrode coating also appears to function to some extent as a collector electrode to collect part of the secondary electrons emitted by the storage target 26, some of which may also be collected by the remaining wall coatings 28, 32 and 36.

It should be noted that a conventional resistive coating 44 of aquadag or similarly conductive material is provided on the interior of a portion of the envelope neck portion 10 and electrically connected to the isolation shield of the writing gun contained in such neck portion so that it serves as an extension of the second anode in such writing gun. A more conductive coating 46 of silver or the like may be provided over the end of conductive coating 44 spaced from the end of the first electrode coating 28, in order to provide a more uniform electrical field at the end of such conductive coating.

A graticule scale 48 is provided on the interior surface of the envelope face plate portion 14 by applying lines of fused glass thereto or scribed notches therein as shown in FIG. 3 and in the abovemcntioned copending application. This internal graticule may be illuminated by a suitable source of light (not shown) positioned outside of the envelope so that the light is transmitted through the surrounding edge of face plate 14 to such graticule scale. Storage target 26 extends over the graticule scale 48 on the interior surface of face plate 14 and, as described below, has a conducting layer extending to the exterior of the envelope. An electrical connection can be made .to such storage target by a connector coating 50 of silver or the like on the exterior surface of the envelope over the glass seal 18.

-One embodiment of the electron gun structure of the storage tube of the present invention is shown in FIGS. 4 and 5 to include a writing gun 52 having a cathode and control grid structure 54, two pair of cross-connected deflection blanking plates 56, a first anode 58 connected to a first anode aperture disc 60, a focusing electrode 62 and a second anode 64. The writing gun extends axially of the tube to direct a focused beam of electrons through a pair of horizontal deflection plates 66 and a pair of vertical deflection plates 68 which are separated by an isolation shield 70 mounted between such pairs of deflection plates. The electrical input signal under investigation is applied to the vertical deflection plates 68 and the horizontal sweep signal to the horizontal deflection plates 66 in a conventional manner through lead pins 20. All of the electrodes of the writing gun 52 may be suitably secured to four glass support rods 72 which are positioned so that they are symmetrically spaced with respect to the electron beam of the Writing gun 52 and extend parallel to the axis of the tube. The support rods are supported by spring members 74 attached to disc 60 and shield 70 so that such spring members engage the neck portion 10 of the envelope.

The storage tube of the present invention is provided with two or more electron flood gun structures 76 in addition to the writing gun 52. The flood gun structures 76 have their cathodes and control grids connected to certain of the base pins 22 in a conventional manner. Such flood guns emit wide beams of flood electrons which substantially uniformly bombard the surface of storage target 26. Each flood gun has a cathode and control grid structure 78, a focusing electrode 80, and an anode 82. The anode is welded to an isolation shield 84 which also serves as a support for the flood guns 76. Each electrode in the flood guns 76 is suitably secured to a pair of short glass support rods 85 and the Whole assembly is attached to the longer support rods 72 by support pins 86 spot welded to isolation shield 84. This isolation shield 84 is in the form of an apertured plate which is bent at an angle along a fold line 87 parallel to the surface of the vertical deflection plates 68. Such fold line passes through the axis of the writing gun so that the axes of the flood guns intersect the axis of the writing gun approximately at the center of the surface of the storage target 26. A rectangular aperture 88 extends through the center of such isolation shield 84 to accommodate the writing beam from the writing gun 52 and a pair of circular openings 90 are provided through such shield on opposite sides of aperture 38 equally spaced therefrom to accommodate the flood beams from the flood guns 76.

Another embodiment of an electron gun structure which may be employed in the storage tube of the present invention is shown in FIGS. 6 and 7 to be similar to the electron gun structure of FIGS. 4 and with the exception that an additional electron gun 92 has been mounted on the isolation shield 84' between the two flood guns 76. A support member 93 is welded in an opening in the shield 84 and has an inclined support portion so that the axis of such additional electron gun intersects the axis of writing gun S2 at substantially the same angle and point as the axes of flood guns 76. This additional electron gun 92 serves as a contrast enhancement gun to increase the contrast of the light image produced on the storage target 26 to the background light emitted by such target as explained below.

Contrast gun 92 is similar in structure to the flood gun 76 in that it includes a cathode and a control grid structure 94., a focusing electrode 96, and an anode 98 all supported in spaced relationship by a pair of short glass support rods 99. The support member 93 is provided with a small opening 1% to accommodate the contrast electron beam which is from about one to ten percent of the current density of the flood gun electron beams so that opening 100 is substantially smaller than openings 90.

One embodiment of the storage target 26 of the present invention is shown in FIG. 8 to include a thin light transparent conductive film 192 on the interior surface of glass face plate 14. This transparent conductive film 102 may be made of tin oxide formed from :stannous chloride, or other suitable material, and is applied to the surface of the face plate in any conventional manner over the graticule scale 48 which may be in the form of fused glass lines 48. The storage dielectric of storage target 26 is a thin layer of phosphor material 104, for example, P1 type phosphor having a chemical designation of Zn SiO :Mn, which is applied over the conductive target electrode film M12 in any suitable manner, such as by water settling of the phosphor on the film or the application of a preformed layer containing phosphor to the film by decalcomania, to provide a thin ise micontinuous integral layer of phosphor having a porous structure and a substantially uni form thickness. The phosphor storage dielectric layer 1 34 is an integral or undivided layer of phosphor particles which are in physical contact with one another, but is sufficiently thin and porous to enable secondary electrons emitted from the bombarded side of the phosphor layer to be transmitted completely through such layer and collected by the target electrode film 102 on the opposite side of the layer. Such thickness is Within that range of thicknesses over which such phosphor material will store a charge image for an indefinite controllable time, with no substantial spreading or migration of the charge image. This critical range of phosphor thicknesses will be discussed in greater detail with reference to FIG. 11.

An electrical connection may be provided to the conductive film 102 from the exterior of the envelope by, for example, extending the conductive film through the glass frit seal 18 so that it may be contacted by the connector coating 50 on the exterior of such envelope. However, this connection may be made in any number of Ways including the provision of a conductive coating of silver or other suitable material on the surface of face plate 14 and extending through the seal 18 into contact with the film 102. Other alternatives include making seal 18 of a conductive glass frit, providing a metal pin through such seal, or using a connector plug similar to plugs 3t 34, 38 and 42. One such connector plug 42 is shown in FIG. 8 as extending through an aperture in the funnel portion 12 and making contact with the fourth electrode wall coating 40. This plug is in the form of a body of ceramic material 1416 similar to the ceramic material of funnel envelope portion 12 and provided with a connector coating 168 of silver or other conductive material on the exterior of such plug body so that coating 108 provides an electrical connection from the exterior of the envelope to the fourth electrode wall coating 49. The plug connector 42 is sealed in the aperture in the funnel portion 12 of the envelope by means of a ceramic-to-metal seal 110.

Another embodiment of the storage target 26' of the present invention is shown in FIG. 9 to be similar to that shown in FIG. 8 except that an intermediate layer of porous glass frit material 114 may be provided between the phosphor storage layer 104 and the conductive film 102. The function of this intermediate glass layer 114 is to increase image contrast by reducing the background light. The glass layer 114 takes over part of the chargestoring function of the phosphor layer 104. It increases the resistivity of the storage dielectric formed by phosphor layer 104 and glass layer 114 and enables the thickness of the phosphor layer 104 to be considerably less than that of the embodiment shown in FIG. 8.

The secondary emission characteristics of a conventional transmission type of dielectric storage target are shown in FIG. 10 by the curves 116 and 118. Curve 116 shows the secondary emission characteristic when the potential of the conventional collector electrod is always maintained slightly more positive than the storage target. This curve begins at a point 120 where the effective or apparent secondary emission ratio (6) is equal to one, and the target surfac voltage (V is about minus one volt with respect to the cathode. Thus it takes a stopping potential (V of about 1 volt on the target to stop ap proximately all the primary electrons emitted by the cathode from reaching such target and to cause them to be transmitted to the collector electrode where they act the same as secondary electrons so that the ratio (5) of secondary electrons collected by the collector electrode to primary electrons emitted by the cathode is one. This stopping potential V is required to prevent primary electrons from reaching the target because such primary electrons are emitted from a heated cathode with a. small average initial velocity due to their energy of thermal emission. As V becomes more positive some primary electrons are transmitted to the target where they are absorbed without producing secondary emission so that 5 decreases until the minimum point 122 on the curve is reached. As V becomes more positive than the voltage at point 122 some of the primary electrons which strike the target have sufficient impact velocity to knock out secondary electrons from the target surface. These secondary electrons are attracted to the collector electrode so that true secondary emission causes 6 to increase until the first crossover point 124 is reached at V zV where again 6:1. The net target current is negative when V V V so that the bombardment of the target by low energy primary electrons in this range tends to increase the negative charge on such target in the direction of the corresponding arrow on curve 116 to cause the target potential to return to point 120 near the cathode potential which is one stable point on the curve 116.

When V is increased above the first crossover point at V 6 increases above one to a maximum at point 126 since the primary electrons strike the target with more energy and each of such primary electrons produces more than one secondary electron at these target voltages. Above the voltage represented by point 126, an increase in V results in a further increase in the velocity of the incident primary electrons and in the number of secondary electrons produced thereby but, according to one explanation, the increased velocity of such primary electrons causes them to penetrate deeper into the target dielectric so that the secondary electrons are absorbed within the target before they can be emitted therefrom. This causes 6 to decrease until it reaches unity at the second crossover point 128 at a target voltage sometimes called the sticking potential (V The net target current is positive when V V V so bombardment of the target by primary electrons in this range of voltages tends to increase the positive charge on such target in the direction of the arrow until it reaches the sticking potential V at point 128 which is the second stable point on curve 116. It should be noted that further increases in V, above V will cause more secondary electrons to be absorbed within the target until substantially none reaches the collector elec trode so that 6 decreases from one toward zero and the net target current is negative in this region. Therefore, when primary electrons strike the target at voltages greater than V they tend to accumulate as a negative charge on such target and drive the target voltage in the direction of the corresponding arrow on curve 116 black down to V at stable point 128.

Curve 1118 shows the secondary emission characteristics of a conventional bistable storage tube when the collector voltage (V is held at a positive voltage in the stable range of operating voltages for the storage dielectric which is often approximately equal to 2V The result of holding the collector voltage at this value while allowing V to vary is to provide a bistable storage tube which can store the electrical charge on the storage target for an indefinite controllable time. Curve 118 is similar to curve 116 at target voltages below a maximum 6 at point 130. However, as V increases above the voltage at point 130 5 decreases rapidly to a second stable point 132 on curve 118 where 6:1. This occurs at a target voltage slightly above V The rapid decrease continues on down to a minimum point 134 where 6 is essentially zero and V is about thirty volts greater than V This rapid change of 6 from maximum point 130 to minimum point 134 is due to the additional suppressor action of the collector electrode on secondary electrons as V approaches and exceeds V Such suppressor action repels such electrons from the collector electrode and aids in the recapture of such secondary electrons by the target. It should be noted that the collector electrode must be about 30 volts with respect to the target before the collector to target voltage reaches the stopping potential of the secondary emission electrons which have a much higher average intial velocity than that of primary electrons produced by thermal emission.

As in conventional bistable storage tubes, the electrons forming the writing beam emitted from the writing gun 52 of the tube of the present invention have suflicient velocity to cause secondary electrons to be emitted from the storage dielectric layer 104 of storage target 26. The result is that the area at the point of impact of such writing beam acquires a net positive voltage producing the charging direction shown above the first crossover voltage (V for such writing beam. The flood electrons emitted from flood guns 7 6 are substantially uniformly distributed over the storage target 26 and, in a stable range, do not have suflicient impact velocity to cause much secondary emission from target areas which have not been struck by the riding beam. Thus, the flood electrons tend to negatively charge those areas of phosphor storage layer 104 which have not been struck by the writing beam until such negative charge is suflicient to repel most of floodelectrons. This target potential corresponds to, or approaches, the first stable point 120 indicated on curve 118 for the flood beam and is approximately equal to the potential of the flood gun cathodes. Point 120 may be higher in voltage if there is substantial conductivity through the target dielectric layer. However, the flood electrons are further accelerated by any positive charge areas on the phosphor storage layer 104 which are present due to the secondary emission caused by bombardment of the writing beam. This additional acceleration provides the flood electrons with sufficient impact velocity to cause greater secondary emission at those target areas already having a positive charge due to the writing beam so that the positive charge of these areas is thereby increased tending to drive the target potential to a voltage corresponding to the second stable point 132 on the curve 118 of FIG. 10. It should be noted that the secondary emission characteristics of the storage target are different for the writing beam and the flood beam since their cathodes are at different voltages.

In actual practice, however, the storage tube of the present invention does not operate entirely in accordance with the secondary emission characteristics shown in FIG. 10. For example, it has been found that the flood electrons do not drive the unwritten areas of the storage layer 104 back to the potential of the flood gun cathodes corresponding to point 12-0 on curve 118 but these unwritten target areas assume a stable voltage believed to be about +40 volts with respect to such cathodes. This accounts for the background illumination. One explanation for this is that the target areas in question are bombarded by positive ions of residual gas to increase the positive charge thereon. It is also possible that the phosphor storage dielectric may become conductive in such target areas so that the flood electrons striking these unwritten areas tend to pass to the conductive film 102 without materially increasing the negative charge of such areas. Another departure from conventional bistable storage tube operation is that the collector can not be varied independently of the target voltage in the storage tube. One explanation for this is that film 102 functions as a collector electrode for secondary electrons transmitted through the porous phosphor layer 104 of FIGS. 8 and 9 and also through the porous glass frit layer 114 of FIG. 9.

Sometypical operating values for the electrodes in the storage tube of FIGS. 1 to 5 are a writing gun cathode Voltage of 3,000 volts, a flood gun cathode voltage of 0 volts, a flood gun grid voltage of l5 volts and a flood gun anode voltage of +200 volts. Anode resistance coating 44 may be operated at +220 volts, the first electrode coating 28 at +200 volts, the second electrode coating 32 at +150 volts, the third electrode coating 36 at +75 volts and the fourth electrode coating 40 at +50 volts. The voltage of the conductive film 102 of the storage target may vary considerably but is typically held at +300 volts. In addition, the total current of both flood gun cathodes may be about 15 milliamperes while the typical total operating current of the conductive film 102 may be from about 2 to 9 milliamperes.

The storage and display characteristics of the storage target of the present invention as a function of the relative thickness of phosphor storage layer 104 are shown in FIG. 11. Briefly, these characteristics are a stable range curve 136, a light image brightness curve 138 and a light image contrast curve 140'. The ordinate of the stable range curve is target voltage stable range and such stable range curve 136 increases from 0 volts to a maximum of about to 120 volts at point 14-2 and then returns to 0 volts at the thickness T which represents a critical thickness above which the phosphor layer 104 will no longer store an electrical charge image for an indefinite controllable time. The absolute value of T is in the neighborhood of .001 to .003 inch depending upon the type of phosphor used and is approximately one-half to one-third of the thickness T of conventional cathode ray tube screens. It should be noted that the storage target of the present invention has an extremely wide stable range of operating voltages up to about volts. In this range the phosphor storage layer will store a charge image without spreading or blurring for an indefinite time so as to provide a bistable storage tube. The stable range of conventional bistable storage tubes is ordinarily defined as that range of collector voltages between the retention threshold voltage where effective bistable storage begins and the fade positive point voltage where such storage stops and the charge becomes uniform over the phosphor layer due to the action of the flood guns. In the present storage tube the stable range voltages are those established between the cathodes of the flood guns 76 and the conductive film 102. The 100 volt stable range referred to is the initial range for new tubes and this large range is important in a bistable storage tube because the width of the stable range reduces with use due to deterioration of the storage dielectric caused by electron bombardment so that thelifetime of the tube is determined to a large extent by the width of its initial stable range.

It has been found that the brightness curve 138 increases from zero continuously and at a rather rapid initial rate as the thickness of the phosphor layer 104 is increased from O to T so that adequate image brightness may be obtained with very thin layers far below that corresponding to the maximum stable range at point 142. However, the contrast curve 140 increases in value from zero at a much lower initial rate than the brightness curve 138 as the phosphor thickness is increased from to T Thus the contrast curve usually determines the lower limit of useful phosphor thicknesess. While the absolute values of brightness and contrast are not indicated, the lower limit thickness at which curve 140 reaches a useful contrast is somewhere to the left of the thickness corresponding to point 142 depending upon the use to which the storage tubes are put. Thus, there is a critical range of thicknesses for the phosphor storage layer 104 beginning above zero and ending below T within which such a layer Will store an electrical charge image for an indefinite controllable time and still give adequate light image brightness and contrast. It will be noted that the brightness curve 138 levels off so that no substantial increase in brightness is obtained by increasing the thickness of the phosphor beyond the thickness T where the curve is sub stantially level. Above the thickness T brightness decreases. Thus the operative range of phosphor thicknesses depends upon both the particular type of phosphor employed and the intended use of the storage tube but, in

general, the lower limit of this range is somewhat greater than Zero and the upper limit is somewhat below one-half the thickness T at which the brightness curve becomes substantially level. For P-l type phosphor this critical range of thicknesses is approximately from .001 to .0025 inch.

It has also been found that the contrast of the light image of the storage tube of the present invention can be enhanced by the use of an additional electron gun similar to the main flood guns 76 and shown as the contrast gun 92 in FIGS. 6 and 7. The cathode of this contrast gun may be operated at a potential of about -10 volts with respect to the cathodes of the main flood guns 76 and at a current of about one percent to ten percent of the main flood gun current. The contrast beam from contrast gun 92, which is similar to the flood beam, reduces the background light emitted by the direct-viewing storage target 26 since the more energetic electrons from the contrast gun tend to drive the surface of the storage layer 104 from the +40 volts obtained by the flood beam electrons toward the 0 volts potential of the cathodes of the flood guns. This same result may be accomplished by applying a l0 volt pulse to the cathodes of the main flood guns about one to ten percent of the time they are turned on.

It has also been found that an improved type of operation of the storage tube results in increasing the stored writing speed of such tube by approximately a factor of two. This increase in writing speed may be accomplished by turning off the flood gun 76 and the contrast gun 92,

if employed, during the writing action of the writing electron beam as it moves across the surface of the storage layer 104. One way of accomplishing this is to apply a large negative pulse of about 200 volts to the control grids of the main flood guns and the contrast gun during the time of such writing action.

In addition, it has been found that the erase operation of a storage tube may be improved so that such tube will erase the written charge image on the storage target more rapidly. This is accomplished by applying a positive voltage .pulse to the conductive target film 102 so that the voltage of such film rapidly increases from its normal operating voltage of about +300 volts to a maximum voltage of about +400 volts above the 0 volt potential of the flood gun cathodes. This increased voltage is maintained for approximately one-fourth second and then reduced in a negative direction to a minimum voltage of about 0 volts, below the stable range, before such voltage is allowed to return more slowly in a positive direction to its normal operating voltage of +300 volts. This operation drives the surface of the dielectric layer 104 remote from the conductive film 102 up to a voltage above the fade positive point and then down to a voltage below the retention threshold voltage by capacitive action without allowing such dielectric surface to follow the potential of the conductive film as it returns to its quiescent operating voltage of +300 volts from 0 volts. This erase pulse has the visual effect of making the storage layer first of a high uniform brightness corresponding to a completely written target condition and then to a level of extremely low uniform brightness corresponding to a completely unwritten target condition at which such target is again ready to receive another written image by the writing beam.

In addition to these improved methods of operation, the storage tube of the present invention can be made so that the phosphor layer 104 is divided int-o two parts of different thickness and occupying different areas, one of which functions as a storage target and the other of which functions as a conventional phosphor screen without bistable charge storage so that the electrical signal under investigation may be first written on the conventional cathode ray tube phosphor screen before it is written on and stored on the storage target portion of such layer. Also, the relatively simple structure of the storage target of the present invention makes possible the use of a color storage tube using different colored phosphor layers or dots as the storage dielectric and employing a conventional shadow mask technique similar to that used in commercial color television receivers of the type using the shadow mask color tube.

-It is also possible to replace the electrodes made up of separate coatings 28, 32, 36, 40 and 44 with a single continuous coating of resistance material whose thickness and resulting resistance varies along the axis of the storage tube envelope so that a potential gradient exists along the surface of such resistive coating. Such resulting potential gradient can replace the progressively decreasing potentials provided by the separate coatings. One such variable thickness resistance coating is shown in the above-mentioned copending US. patent application, Serial No. 132,915. Also, it is possible to employ a conventional helical strip wall coating such as that employed as the post deflection anode of a conventional cathode ray tube in place of the separate wall coating electrodes discussed above. With either of these additional types of wall coating electrodes it may be desirable to apply a more positive voltage to the end of such electrodes adjacent the flood guns during the storage period than during a writing operation in order to prevent such electrodes from repelling all or a major portion of the primary flood electrons back to the flood gun anode before they can reach the storage target due to the large negative voltage applied to such end of such electrodes for acceleration of the writing beam during such writing operation. More positive voltages can be applied to such electrodes during a writing operation to help accelerate the electrons of the writing beam to thereby obtain the advantages of conventional post-acceleration.

Obviously, the storage tube of the present invention may be used as an electrical readout device as well as a direct-viewing type storage tube by connecting the conductive film 102 to the control grid of a TV monitor tube and applying the raster signals of such monitor tube to the deflection plates 66 and 68 in a conventional manner since the writing gun 52 can also be used as the reading gun of such a storage tube. In addition to this a separate reading gun can be employed within the tube envelope in order to increase the speed of operation of such an electrical readout storage tube. Furthermore, two writing guns may be employed in the storage tube so that it may function as a dual beam type cathode ray tube for the display of two different electrical input signals. Another device which might be employed with the storage tube of the present invention is a beam rotator made up of a coil of wire positioned so that it surrounds the funnel portion of the tube envelope adjacent the storage target in order to rotate the writing beam so that its vertical and horizontal deflection positions are in alignment with the graticule lines on the interior of the face plate portion of such envelope. Such a beam rotator is not necessary when a conventional round tube is used which has an external graticule because then it is possible to merely rotate the tube so that the writing beam is in alignment with the graticule scale.

From the above it should be obvious to one having ordinary skill in the art that various changes may be made in the detail of the above described preferred embodiments of the present invention. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. A storage target, comprising:

a conductive substrate body providing a target electrode;

and

a charge storage means including an integral semicontinuous dielectric layer of phosphor material supported on one side of said substrate body and having a thickness that is within the range of thicknesses over which said phosphor material is capable of storing an electrical charge image produced on said layer and Will emit a light image corresponding to said charge image for an indefinite time when bombarded by electrons, said integral phosphor layer within said range of thicknesses being sutficiently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer.

2. A storage target, comprising:

a substrate body of electrical insulative material which is light transparent and has an electrically conductive region on one side thereof; and

a charge storage dielectric layer of phosphor material supported on said one side of said substrate body, said storage layer being an intergral layer having a thickness that is within the range of thicknesses over which said phosphor will store an electrical charge image produced on said layer for an indefinite time and will emit a light image corresponding to said charge image when bombarded by electrons, said range of thicknesses existing below one-half the thickness at which no substantial increase in the brightness of said light image is obtained by further increases in the thickness of'said phosphor storage layer, said integral phosphor layer within said range of thicknesses being sufiiciently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the l 5 phosphor layer and collected by said conductive region on the opposite side of said phosphor layer.

3. A direct viewing storage target for use in a cathode ray storage tube, comprising:

a substrate body of electrical insulative material which is light transparent;

a light transparent film of electrical conductive material supported on one side of said substrate body and forming a target electrode; and

a charge storage dielectric layer of phosphor material supported on said one side of said substrate body over said conductive film, said storage layer being an integral semicontinuous layer having a thickness that is within the range of thicknesses over which said phosphor will store an electrical charge image produced on said layer for an indefinite controllable time and will emit a visible light image corresponding to said charge image, when bombarded by electrons, said integral phosphor layer within said range of thicknesses being sufiiciently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer.

4. A direct viewing storage target for use in a bistable cathode ray storage tube, comprising:

a substrate body of glass insulative material which is light transparent;

a light transparent film of electrical conductive material supported on one side of said substrate body and forming a target electrode;

a light transparent coating of porous glass frit insulative material secured to said one side of said substrate body over said conductive film; and

a charge storage dielectric layer of phosphor material supported on said one side of said substrate body over said insulative coating and said conductive film, said storage layer being a semicontinuous integral layer having a thickness that is within the range of thicknesses over which said phosphor will store an electrical charge image for an indefinite controllable time over a stable range of collector voltages and will emit a visible light image corresponding to said charge image, when bombarded substantially uniform by electrons, said integral phosphor layer within said range of thicknesses being sufficiently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer.

5. A storage tube, comprising:

an evacuated envelope;

a storage target mounted inside said envelope, said target including a storage dielectric layer of phosphor material supported on a light transparent support member over an electrically conductive region on one side of said support member forming a target electrode so that said storage layer is an integral semicontinuous layer having a thickness that is within the range of thicknesses over which said phosphor material will store an electrical charge image for an unlimited time when bombarded by electrons, said integral phosphor layer within said range of thicknesses being sutficiently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer;

writing means for generating a first electron beam of high velocity electrons and for directing said first beam onto said storage layer to form said charge image on said storage target;

holding means for producing a second electron beam of low velocity electrons and for directing said second beam onto said storage layer to retain said charge image formed by said first beam on said storage target, and to erase said charge image when said first beam no longer bombards said target; and

focusing means mounted inside said envelope for focusing said second electron beam onto said phosphor layer and collected by the target electrode on the opposite side of said phosphor layer;

writing means for generating, focusing and deflecting a first electron beam which may be used as a writing beam to form said charge image on said storage target by bombardment of a portion of said storage layer, layer in response to an electrical image signal applied 6, A direct viewing cathode ray storage tube, comto the beam deflecting means; Prlslflg! holding means for producing a second electron beam an evacuated envelope of electrical insulative material which may be employed as a holding beam to retain having a light transparent face plate portion; 10 said charge image formed by said first beam on said a storage target mounted on said face plate portion of storage target and to erase said charge image by said envelope, said target including a light transparent charging portions of said storage layer to one of two film of electrical conductive material supported on bistable states of electrical potential, when said first the interior surface of said face plate portion and beam no longer bombards said target; and having an electrical connection to the exterior of said means mounted inside said envelope adjacent said storenvelope so that said film may be connected to a age target including at least one conductive coating source of electrical potential to provide a target elecon the wall f said envelope for focusing, collimating trode, and a storage dielectric layer of phosphor and uniformly distributing said second beam over material supported over said conductive film so that said phosphor layer. said storage layer is an integral layer of phosphor 8. A direct viewing bistable cathode ray storage tube, having a thickness that is within the range of thickcomprising: nesses over which said phosphor exhibits an unlimited an evacuated envelope of electrical insulative material image persistence so that said storage layer will store having a light transparent face plate portion; an electrical charge image produced on said storage a storage target mounted on said face plate portion layer for an indefinite controllable time and will emit inside said envelope, said target including a light a light image corresponding to said charge image, transparent film of electrical conductive material when bombarded by electrons, said integral phosphor supported on the interior surface of said face plate layer within said range of thicknesses being sufiiportion and having an electrical connection to the ciently thin and porous to enable secondary electrons exterior of said envelope so that said film may be emitted fro-m the bombarded side of said phosphor connected to a source of electrical potential in order layer to be transmitted through the phosphor layer to form a target electrode, and a storage dielectric and collected by the target electrode on the opposite layer of phosphor material supported over said conside of said phosphor layer; ductive film so that said storage layer is a semiwriting means for generating a first electron beam of continuous integral layer of phos hor having a subhigh velocity electrons and for directing said first stantially uniform thickness that is within the range beam onto said storage layer to form said charge of thicknesses over which said phosphor will store image and said light image on said storage target, and an electrical charge image for an indefinite conholding means for producing a second electron beam of trollable time and will emit a visible light image low velocity electrons and for directing said second corresponding to said charge image, when bombarded beam onto said storage layer to retain said charge by electrons, said integral phosphor layer within said and light images formed by said first beam on said range of thicknesses being sufiiciently thin and porous storage target and to erase said charge and light to enable secondary electrons emitted from the images, when aid first beam no longer bombards bombarded side Of said phosphor layer '[0 be transid target mitted through the phosphor layer and collected by 7. A direct viewing bistable cathode ray storage tube, the target electrode on the Opposite Side of Said comprising: P p y an evacuated envelope of electrical insulative material Writing means for generating, focusing and deflecthaving a light transparent face plate portion; 21 first electron beam of g elfictron finergy a storage target mounted on said face plate portion in- Which y be used as a Writing beam to form Said side said envelope, said target including a light transcharg image 011 Said Storage target y bombafding parent film of electrical conductive material supa Portion of the Outer sufface of said Storage y ported on the interior surface of said face plate porficod g means prodll'clng a Second Plectron beam tion, and having an electrical connection to the ex- Y 165$ electron e ergy than said first beam, terior of said envelope so that said film may be con- W may b6 employed as a h ing eam by causnected to a source of electrical potential to provide mg H to bombard Substantlany the entlre Outer surface of said storage layer so that the surface a target electrode, and a storage dielectric layer of f h n d f luminescent phosphor material supported over said portloils of Sal outer Sur are c ar-ae to One-O two bistable states of electrical potential depending conductlle film,sc that Said storfjlge layer 13 upon whether said surface portions have been bomteg'ral sem{connnuous f t having a substanmmy barded by said first electron beam, to retain said uniform thickness that is within the range of thickcharge image formed by said first beam on Said nesses over which said phosphor has unlimited image Storage target and to erase said charge image, when Persistence 50 that Said Storagfi layer Will Store all said first beam no longer bombards said target; and electrical charge image for an indefinte controllable electrode means including a plurality of spaced contime and will emit a visible light image correspondductive coatings on the inside of said envelope for ing to said charge image when bombarded by elecfocusing, collimating and uniformly distributing said trons, said range of thickness lying below one-half second electron beam over said phosphor layer. the thickness at which no substantial increase in 9. A direct viewing bistable storage tube, comprising: brightness is obtained by further increases in the an evacuated envelope of electrical insulative material thickness of said phosphor storage layer, said integral having a light transparent face plate portion; phosphor layer within said range of thicknesses being a storage target mounted on said face plate portion insufiiciently thin and porous to enable secondary elecside said envelope, said target including a light transtrons emitted from the bombarded side of said phosparent film of electrical conductive material supphor layer to be transmitted through the phosphor ported on the interior surface of said face plate portion and having an electrical connection to the exterior of said envelope so that said film may be connected to a source of electrical potential to form a target electrode, and a storage dielectric layer of phosphor material supported over said conductive film so that said storage layer is an integral semicontinuous layer of phosphor having a substantially uniform thickness that is Within the range of thicknesses over which said phosphor will store an electrical charge image for an indefinite controllable time and will emit a light image corresponding to said charge image, when bombarded by electrons, said integral phosphor layer within said range of thicknesses being sufliciently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer;

writing means for generating, focusing and deflecting a first electron beam which may be used as a writing beam to form said charge image on said storage target;

holding means for producing a second electron beam which may be employed as a holding beam to retain said charge image formed by said first beam on said storage target and to erase said charge image by bombarding said storage layer substantially uniformly when said first beam no longer bombards said tarmeans mounted inside said envelope adjacent said storage target for focusing, collimating and uniformly distributing said second beam over phosphor layer; and

means for producing a third electron beam which may be used to increase the contrast of said light image emitted by said storage target, said third beam having electrons with more energy than those of said second beam so that the electrons of said third beam bombard said target at greater impact velocities than the electrons of said second beam, with the number of electrons of said third beam being substantially less than the number of electrons of said second beam Which strike said target during the time of said light image.

10. A direct viewing bistable storage tube, comprising:

an evacuated envelope having a body portion of ceramic insulative material and a light transparent, flat face plate portion of glass insulative material;

a storage target mounted on said face plate portion inside said envelope, said target including a light transparent film of electrical conductive material supported on the interior surface of said face plate portion and having an electrical connection to the exterior of said envelope so that said film may be connected to a source of electrical potential to form a target electrode, and a storage dielectric layer of phosphor material supported over said conductive film so that said storage layer is a semicontinuous integral layer of phosphor having a substantially uniform thickness that is within the range of thicknesses over which said phosphor has unlimited image persistence so that said storage layer will store an electrical charge image for an indefinite controllable time and will emit a visible light image corresponding to said charge image, when bombarded by electrons, said integral phosphor layer within said range of thicknesses being sufiiciently thin and porous to enable secondary electrons emitted from the bombarded side of said phosphor layer to be transmitted through the phosphor layer and collected by the target electrode on the opposite side of said phosphor layer;

writing gun means for generating, focusing and deflecting a first electron beam which may be used as a writing beam to form said charge image on said storage target by bombardment of said phosphor layer to produce positive written portions on said phosphor layer;

flood gun means for producing a second electron beam which may be employed as a holding beam to retain said charge image formed by said first beam on said storage target and to erase said charge image when said first beam no longer bombards said target;

electrode means mounted inside said envelope for focusing, collimating and uniformly distributing said second beam over said phosphor layer, said electrode means including at least one conductive coating on the interior surface of said body portion of said envelope electrically connected to the exterior of said envelope; and

means for producing a third electron beam which may be used to increase the contrast of said light image emitted by said storage target, said third beam being emitted from a source having a more negative electrical potential than the source of said second beam so that the electrons of said third beam bombard said target at greater impace velocities than the electrons of said second beam to charge the unwritten portions of said phosphor layer more negative than they are charged by said second beam, and with the number of the primary electrons of said third beam being substantially less than the number of primary electrons of said second beam which strike said target during the time of said light image.

11. An electron image storage tube, comprising:

an evacuated envelope;

a storage target mounted within said envelope including a substrate body having an electrically conductive surface, and a storage dielectric layer supported on said body over said conductive surface;

said dielectric layer being an integral layer of phosphor particles having a thickness that is within the range of thicknesses over which said phosphor layer is capable of bistable storage, and said phosphor layer within said range of thicknesses being of a sufficiently thin and porous structure to enable secondary electrons emitted from one side of said layer to be trans mitted through said layer and collected by said conductive surface on the opposite side of said layer;

writing means for producing a charge image on said phosphor layer; and

holding means for bombarding said phosphor layer with low velocity electrons to cause secondary electrons to be emitted from the phosphor layer and bistable storage of said charge image.

References Cited by the Examiner OTHER REFERENCES RCA Review (Viewing Storage Tubes With Half-Tone Display), by Knoll et al., December 1953.

References Cited by the Applicant UNITED STATES PATENTS 6/1938 Gabor. 11/1957 Haelf.

JAMES W. LAWRENCE, Primary Examiner.

ARTHUR GAUSS, Examiner. R. SEGAL, S. CHATMON, JR., Assistant Examiners. 

1. A STORAGE TARGET, COMPRISING: A CONDUCTIVE SUBSTRATE BODY PROVIDING A TARGET ELECTRODE; AND A CHARGE STORAGE MEANS INCLUDING AN INTEGRAL SEMICONTINUOUS DIELECTRIC LAYER OF PHOSPHOR MATERIAL SUPPORTED ON ONE SIDE OF SAID SUBSTRATE BODY AND HAVING A THICKNESS THAT IS WITHIN THE RANGE OF THICKNESS OVER WHICH SAID PHOSPHOR MATERIAL IS CAPABLE OF STORING AN ELECTRIC CHARGE IMAGE PRODUCED ON SAID LAYER AND WILL EMIT A LIGHT IMAGE CORRESPONDING TO SAID CHARGE IMAGE FOR AN INDEFINITE TIME WHEN BOMBARDED BY ELECTRONS, SAID INTEGRAL PHOSPHOR LAYER WITHIN SAID RANGE OF THICKNESSES BEING SUFFICIENTLY THIN AND POROUS TO ENABLE SECONDARY ELECTRONS EMITTED FROM THE BOMBARDED SIDE OF SAID PHOSPHOR LAYER TO BE TRANSMITTED THROUGH THE PHOSPHOR LAYER AND COLLECTED BY THE TARGET ELECTRODE ON THE OPPOSITE SIDE OF SAID PHOSPHOR LAYER. 